Optical translation measurement

ABSTRACT

A method for determining the relative motion of a surface with respect to a measurement device, comprising: 
     illuminating the surface with incident illumination, having a coherence length, such that the illumination is reflected from portions of the surface, wherein at least part of at least one of the incident and reflected illumination passes through a partially transmitting object that is part of the measuring device; 
     detecting the illumination reflected from the surface, to generate a detected signal; and 
     determining the relative motion of the surface parallel to the surface, from the detected signal, 
     wherein the object and the surface are situated within a distance that is less than the coherence length of the detected illumination.

FIELD OF THE INVENTION

The present invention is related to the field of velocity andtranslation measurement and more particularly to methods and apparatusfor the non-contact optical measurement translation and velocity.

BACKGROUND OF THE INVENTION

Various optical methods for the measurement of the relative velocityand/or motion of an object with respect to a measurement system exist.Each method and apparatus is characterized by the kinds of objects andthe kinds of motions on which it operates.

The kind of measurable objects may be broadly divided into severalgroups, including:

A specially patterned object, for example, a scale.

A reflecting surface, for example, a mirror.

A small particle (or few particles), for example precursor particles orbubbles suspended in fluid.

An optically contrasting surface, for example, a line or dot pattern.

An optically diffuse object, for example, blank paper.

The kind of measurable motions may be broadly divided into severalgroups, including:

Axial movement toward or away from the measuring device.

Transverse (or tangential) motion, where the spacing between themeasuring device and the object is essentially constant.

Rotational motion, where the object orientation with respect to themeasurement device is changing.

It is also useful to classify the measurement devices according to thenumber of simultaneously obtainable measurement directions (one, two orthree dimensional) and the number of critical components (light sources,light detectors, lenses, etc.).

It should be noted that a specific method may be related to more thanone group in the above classification schemes.

A number of systems capable of non-contact measurement of the transversevelocity and/or motion of objects using optical means have beenreported. These methods can include Speckle Velocimetry methods andLaser Doppler Velocimetry methods. Other methods of interest forunderstanding the present invention are Image Velocimetry methods,homodyne/heterodyne Doppler Velocimetry or Interferometry methods andOptical Coherence Tomography (OCT).

Speckle Velocimetry methods are generally based on the followingoperational principles:

A coherent light source illuminates the object the motion of which needsto be measured.

The illuminated object (generally an opaque surface) consists ofmultiple scattering elements, each with its own reflection coefficientand phase shift relative to the other scattering elements.

The individual reflection coefficients and phase shifts aresubstantially random. At a particular point in space, the electric fieldamplitude of the reflection from the object is the vector sum of thereflections from the illuminated scattering elements, with an additionalphase component that depends on the distance between the point and eachelement.

The light intensity at a point will be high when contributions generallyadd in phase and low when they generally add out of phase (i.e.,subtract).

On a planar surface (as opposed to a point), an image of random brightand dark areas is formed since the relative phase retardation of thesource points depends on the location in the plane. This image is calleda “speckle image,” composed of bright and dark spots (distinct“speckles”).

The typical “speckle” size (the typical average or mean distance for asignificant change in intensity) depends primarily on the lightwavelength, on the distance between the object and the speckle imageplane and on the size of the illuminated area.

If the object moves relative to the plane in which the speckle image isobserved, the speckle image will move as well, at essentially the sametransverse velocity. (The speckle image will also change since somescatterers leave the illuminated area and some enter it).

The speckle image is passed through a structure comprising a series ofalternating clear and opaque or reflecting lines such that the speckleimage is modulated. This structure is generally a pure transmissiongrating, and, ideally is placed close to the detector for maximumcontrast.

The detector translates the intensity of the light that passes throughthe structure to an electrical signal which is a function of theintensity (commonly a linear function).

When the object moves with respect to the measuring device, the speckleimage is modulated by the structure such that the intensity of lightthat reach the detector is periodic. The period is proportional to theline spacing of the structure and inversely proportional to the relativevelocity.

By proper signal analysis, the oscillation frequency can be found,indicating the relative velocity between the object and the measurementdevice.

For these methods high accuracy frequency determination requires a largedetector while high contrast in the signal requires a small detector. Apaper by Popov & Veselov, entitled “Tangential Velocity Measurements ofDiffuse Objects by Using Modulated Dynamic Speckle” (SPIE0-8194-2264-9/96), gives a mathematical analysis of the accuracy ofspeckle velocimetry.

U.S. Pat. No. 3,432,237 to Flower, el. al. describes a specklevelocimetry measuring system in which either a transmission pattern or apin hole is used to modulate the speckle image. When the pin-hole isused, the signal represents the passage of individual speckles acrossthe pin hole.

U.S. Pat. No. 3,737,233 to Blau et. al. utilizes two detectors in anattempt to solve the problem of directional ambiguity which exists inmany speckle velocimetric measurements. It describes a system having twodetectors each with an associated transmission grating. One of thegratings is stationary with respect to its detector and the other moveswith respect to its detector. Based on a comparison of the signalsgenerated by the two detectors, the sign and magnitude of the velocitymay be determined.

U.S. Pat. No. 3,856,403 to Maughmer, et al. also attempts to avoid thedirectional ambiguity by providing a moving grating. It provides a biasfor the velocity measurement by moving the grating at a velocity higherthen the maximum expected relative velocity between the surface and thevelocimeter. The frequency shift reduces the effect of changes in thetotal light intensity (DC and low-frequency component), thus increasingthe measurement dynamic range and accuracy.

PCT publication WO 86/06845 to Gardner, et al. describes a systemdesigned to reduce the amplitude of DC and low frequency signalcomponents of the detector signal by subtracting a reference sample ofthe light from the source from the speckle detector signal. Thereference signal is proportional to the total light intensity on thedetector, reducing or eliminating the influence of the total intensityvariations on the measurement.

This reference signal is described as being generated by using abeam-splitter between the measured surface and the primary detector byusing the grating that is used for the speckle detection also as abeam-splitter (using the transmitted light for the primary detector andthe reflected light for the reference detector) or by using a second setof detectors to provide the reference signal. In one embodimentdescribed in the publication the two signals have the same DC componentand opposite AC components such that the difference signal not onlysubstantially removes the DC (and near DC) components but alsosubstantially increases the AC component.

In U.S. Pat. No. 4,794,384, Jackson describes a system in which aspeckle pattern reflected from the measured surface is formed on a 2 DCCD array. The surface translation in 2 dimensions is found usingelectronic correlation between successive images. He also describes anapplication of his device for use as a “padless optical mouse.”

Image velocimetry methods measure the velocity of an image across theimage plane. The image must include contrasting elements. A line pattern(much like a grating) space-modulate the image, and a light-sensitivedetector is measuring the intensity of light that pass through thepattern. Thus, a velocity-to-frequency relation is formed between theimage velocity and the detector AC component. Usually, the line patternmoves with respect to the detector so that the frequency is biased.Thus, the direction ambiguity is solved and the dynamic range expanded.

A paper by Li and Aruga, entitled “Velocity Sensing by Illumination witha Laser-Beam Pattern” (Applied Optics, 32 p.2320, 1993) describes imagevelocimetry where the object itself is illuminated by a periodic linestructure (instead of passing its image through such a pattern). Theline pattern is obtained by passing an expanded laser beam throughperiodic transmission grating (or line structure). According to thesuggested method the object still needs to have contrasting features.

There exist a number of differences between Image Velocimetry (IV) andSpeckle Velocimetry (SV). In particular, in SV the random image isforced by the coherent light source, whereas in IV an image with propercontrasting elements is already assumed. Furthermore, in SV thetangential velocity of the object is measured, whereas in IV the angularvelocity is measured (the image velocity in the image plane isproportional to the angular velocity of the line of sight).

In U.S. Pat. No. 3,511,150 to Whitney et. al., two-dimensionaltranslating of line patterns creates a frequency shift. A singlerotating circular line pattern creates all the necessary translatingline patterns at specific elongated apertures in a circular mask. Thefrequency shift is measured on-line using an additional detectormeasuring a fixed image. The line pattern is divided to two regions,each one adapted for the measurement of different velocity range. Thesystem is basically intended for image motion compensation in order toreduce the image blur in aerial photography. Also, it is useful formissile homing heads.

U.S. Pat. No. 2,772,479 to Doyle describes an image velocimetry systemwith a frequency offset derived from a grating on a rotating belt.

Laser Doppler Velocimeters generally utilize two laser beams formed bysplitting a single source which interfere at a known position. Alight-scattering object that passes through the interfering spacescatters light from both beams to a detector. The detector signalincludes an oscillating element with frequency that depends on theobject velocity. The phenomena can be explained in two ways. Oneexplanation is based on an interference pattern that is formed betweenthe two beams. Thus, in that space the intensity changes periodicallybetween bright and dark planes. An object passing through the planesscatters the light in proportion to the light intensity. Therefore, thedetected light is modulated with frequency proportional to the objectvelocity component perpendicular to the interference planes. A secondexplanation considers that an object passing through the space in whichboth light beams exist, scatters light from both. Each reflection isshifted in frequency due to the Doppler effect. However, the Dopplershift of the two beams is different because of the different angles ofthe incident beams. The two reflections interfere on the detector, suchthat a beat signal is established, with frequency equal to thedifference in the Doppler shift. This difference is thus proportional tothe object velocity component perpendicular to the interference planes.

It is common to add a frequency offset to one of the beams so that zeroobject velocity will result in a non-zero frequency measurement. Thissolves the motion direction ambiguity (caused by the inability todifferentiate between positive and negative frequencies) and it greatlyincreases the dynamic range (sensitivity to low velocities) by producingsignals far from the DC components. The frequency offset also has otheradvantages related to signal identification and lock-on.

U.S. Pat. No. 5,587,785 to Kato, et. al. describes such a system. Thefrequency offset is implemented by providing a fast linear frequencysweep to the source beam before it is split. The method of splitting issuch that a delay exists between the resulting beams. Since thefrequency is swept, the delay results in a fixed frequency differencebetween the beams.

Multiple beams with different frequency offsets can be extracted byfurther splitting the source with additional delays. Each of thesedelays is then used for measuring a different velocity dynamic range.

A paper by Matsubara, et al., entitled “Simultaneous Measurement of theVelocity and the Displacement of the Moving Rough Surface by a LaserDoppler Velocimeter” (Applied Optics, 36, p. 4516, 1997) presents amathematical analysis and simulation results of the measurement of thetransverse velocity of a rough surface using an LDV. It is suggestedthat the displacement along the axial axis can be calculated frommeasurements performed simultaneously by two detectors at differentdistances from the surface.

In Homodyne/Heterodyne Doppler Measurements, a coherent light source issplit into two beams. One beam (a “primary” beam) illuminates an objectwhose velocity is to be measured. The other beam (a “reference” beam) isreflected from a reference element, usually a mirror, which is part ofthe measurement system. The light reflected from the object and from thereference element are recombined (usually by the same beam splitter) anddirected to a light-sensitive detector.

The frequency of the light reflected from the object is shifted due tothe Doppler effect, in proportion to the object velocity component alongthe bisector between the primary beam and the reflected beam. Thus, ifthe reflected beam coincides with the primary beam, axial motion isdetected.

The detector is sensitive to the light intensity, i.e.—to the square ofthe electric field. If the electric field received from the referencepath on the detector is E₀(t)=E₀ cos(ω₀t+φ₀) and the electric fieldreceived from the object on the detector is E₁(t)=E₁ cos(ω₁t+φ₁), thenthe detector output signal is proportional to (E₀+E₁)² =E₀ ² +E₀E₁+E₁ ².

The first term on the right side of the equation is averaged by thedetector time-constant and results in a DC component. The intensity ofthe reference beam is generally much stronger than that of the lightreaching the detector from the object, so the last term can usually beneglected. Developing the middle term:

E ₀ E ₁ =E ₀ E ₁cos(ω₀ t+φ ₀)cos(ω₁ t+φ ₁) =½E ₀ E ₁[cos((ω₀+ω₁)t+φ₀+φ₁)+cos((ω₀−ω₁)t+φ ₀−φ1)]

From this equation it is evident that E₀E₁ includes two oscillatingterms. One of these terms oscillates at about twice the opticalfrequency, and is averaged to zero by the detector time-constant. Thesecond term oscillates with frequency ω₀−ω₁, i.e.—with the samefrequency as the frequency shift due to the Doppler effect. Thus, thedetector output signal contains an oscillating component with frequencyindicative of the measured velocity.

It is common to add a frequency offset to the reference beam. When sucha frequency bias is added, it is termed Heterodyne Detection.

U.S. Pat. No. 5,588,437 to Byrne, et al. describes a system in which alaser light source illuminates a biological tissue. Light reflected fromthe skin surface serves as a reference beam for homodyne detection oflight that is reflected from blood flowing beneath the skin. Thus, theskin acts as a diffused beam splitter close to the measured object. Anadvantage of using the skin as a beam splitter is that the overallmovement of the body does not effect the measurement. Only the relativevelocity between the blood and the skin is measured.

The arrangement uses two pairs of detectors. Each pair of detectors iscoupled to produce a difference signal. This serves to reduce the DC andlow-frequency components interfering with the measurement. A beamscanning system enables mapping of the two-dimensional blood flow.

In Optical Coherence Tomography (OCT), a low-coherence light source(“white light”) is directed and focused to a volume to be sampled. Aportion of the light from the source is diverted to a reference pathusing a beam-splitter. The reference path optical length iscontrollable. Light reflected from the source and light from thereference path are recombined using a beam-splitter (conveniently thesame one as used to split the source light). A light-sensitive detectormeasures the intensity of the recombined light. The source coherencelength is very short, so only the light reflected from a small volumecentered at the same optical distance from the source as that of thereference light coherently interferes with the reference light. Otherreflections from the sample volume are not coherent with the referencelight. The reference path length is changed in a linear manner(generally periodically, as in sawtooth waveform). This allows for asampling of the material with depth. In addition a Doppler frequencyshift is introduced to the measurement, allowing for a clear detectionof the coherently-interfering volume return with a high dynamic range.

In conventional OCT, a depth profile of the reflection magnitude isacquired, giving a contrast image of the sampled volume. In moreadvanced OCT, frequency shifts, from the nominal Doppler frequency, aredetected and are related to the magnitude and direction of relativevelocity between the sampled volume (at the coherence range) and themeasurement system.

U.S. Pat. No. 5,459,570 to Swanson, et al. describes a basic OCT systemand numerous applications of the system.

A paper by Izatt et al., entitled “In Vivo Bidirectional Color DopplerFlow Imaging of Picoliter Blood Volumes Using Optical CoherenceTomography” (Optics Letters 22, p.1439, 1997) describes anoptical-fiber-based OCT with a velocity mapping capability. Anoptical-fiber beam-splitter is used to separate the light paths beforethe reflection from the sample in the primary path and from the mirrorin the reference path and combine the reflections in the oppositedirection.

A paper by Suhara et al., entitled “Monolithic Integrated-OpticPosition/Displacement Sensor Using Waveguide Gratings and QW-DFB Laser”(IEEE Photon. Technol. Lett. 7 p.1195, 1995) describes a monolithic,fully integrated interferometer, capable of measuring variations in thedistance of a reflecting mirror from the measuring device. The deviceuses a reflecting diffraction element (focusing distributed Braggreflector) in the light path from the source as a combined beam-splitterand local oscillator reflector. Direction detection is achieved by anarrangement that introduces a static phase shift between signals of thedetectors.

Each of the above referenced patents, patent publications and referencesis incorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention in its broadest form provides an OpticalTranslation Measurement (OTM) method and device, capable of providinginformation indicative of the amount and optionally the direction ofrelative translation between the device and an adjacent object.Preferably, the object is at least partly rough and is closely spacedfrom the device. As used herein, the terms “rough” or “diffuse” meanoptically irregular or non-uniform. In particular, the object may have adiffuse opaque or semi-transparent surface such as a paper. Thisspecification deals mainly with determining the translation or velocityof such diffuse surfaces. However, it should be understood that many ofthe methods of the invention may also be applicable to determination oftranslation of other types of objects such as small scatteringparticles, possibly suspended in fluid. Translation of the object meansthat its rotation in space may be neglected.

In a first aspect of some preferred embodiments thereof, the inventionprovides heterodyne or homodyne detection of non-Doppler,non-speckle-image signals derived from changes in the phase and/or theamplitude of reflection from an optically irregular surface.

In a second aspect of some preferred embodiments of the invention,applicable to various methods of motion or velocity detection, a systemis provided in which a reflector which reflects part of the incidentlight is placed next to the surface whose motion is to be measured. Thereflector provides a local oscillator signal which is inherentlycoherent with the light which is reflected from the surface. This aspectof the invention is applicable to both Doppler and non-Doppler methodsof motion detection.

In a preferred embodiment of the invention, the partial reflector is anapertured reflector in which the illumination of the surface whosemotion is measured pass through the aperture. In a preferred embodimentof the invention, the partial reflector covers a portion of the measuredsurface and has a substantial amount of transmission. In this preferredembodiment of the invention, the reflections from the surface passthrough the partial reflector. A combination of apertureing and partialtransmission is often useful, especially in preferred embodiments of theinvention which utilize the third aspect of the invention.

In a third aspect of some preferred embodiments of the invention, anon-symmetrical transmission pattern is provided to aid in determiningthe direction of motion of the surface.

In a fourth aspect of some preferred embodiments of the invention, aphase shift is introduced between the reflection from the partialreflector and the reflection from the surface. This phase shift enablesthe determination of the direction of motion, increases the dynamicrange and improves the signal-to-noise ratio.

This phase shift may, in some preferred embodiments of the invention, bedynamic, i.e., time varying. Such phase variations are convenientlyperformed by moving the reflector either perpendicularly to the surfaceor parallel to the surface or a combination of both. Also, the movementmay be of a pattern on the reflector, e.g.—the movement of a standingwave acting as a grating in a Surface Acoustic Wave (SAW) component. Inthis respect it is the pattern on the reflector that moves, and not thewhole reflector. Alternatively, the phase shift is introduced byperiodically varying the optical path length between the reflector andthe surface, e.g. by inserting a piezo-electric material in the opticalpath.

The phase shift may also be a static phase shift. Conveniently, thisstatic phase shift is accompanied by a change in polarization of one ofthe beams (or a part of the energy in the beam). The direction of motionis determined by a measurement of the phase change and more particularlyby measurement of the sign of the phase change. In a preferredembodiment of the invention, the phase of a portion of a beam whosephase is not changed is compared with the phase of a portion which ischanged, to determine the direction of motion.

A fifth aspect of some preferred embodiments of the invention providesfor Doppler based detection of motion of a surface in a directionparallel to the surface. In this aspect of the invention, a single beammay be incident at an angle to the surface or even perpendicular to thesurface.

A sixth aspect of some preferred embodiments of the invention providesfor simultaneous two or three dimensional translation detection using asingle illuminating beam and a single reflector to provide localoscillator reference beams. In a preferred embodiment of the invention,the signal generated by a single detector is used to determine thetranslation in two dimensions.

In a seventh aspect of some preferred embodiments of the invention, onlya single spatial frequency of the light reflected from the surface isutilized in measurement. Preferably, a spatial filter is provided suchthat the illumination is reflected from the surface such thatsubstantially only a single spatial frequency of the reflected radiationis detected by the detector.

In some preferred embodiments of the invention which incorporate thisaspect of the invention, the spatial filter comprises a lens having afocal point and a pinhole which is placed at the focal point of thelens.

Preferably, the illumination of the surface is collimated and thespatial filter filters the reflected illumination such that onlyradiation reflected from the surface substantially in a single directionis detected by the detector.

A device, according to a preferred embodiment of the invention, includesa light source, a grating, a spatial filter, a photo-detector, andsignal processing electronics. The light source provides at leastpartially coherent radiation, which is directed toward the surface. Anoptical grating is placed between the surface and the light source,preferably close to the surface. The light reflected from the surfaceinterferes with the light that is reflected from the grating itself. Thedetector signal includes an oscillating component, that isrepresentative of the surface translation relative to the opticaldevice. The interference may take place with the normal reflection fromthe grating or with light diffracted at any of the grating orders. Mostpreferably, the light passes through a spatial filter prior to detectionby the detector. Two dimensional translation measurement may be achievedby using orthogonal reflection orders from a two-dimensional grating orby utilizing two separate gratings for the two directions. A thirddimension may be deduced by vector calculation of the translationsmeasured in two different orders at the same axis (e.g.—0 and 1, 1 and2, −1 and 1, etc.), or without an additional detector, simultaneouslyusing different signal analysis techniques on the same signal.

Optional detection of the direction of translation (as opposed to it'sabsolute magnitude) is preferably achieved by modulating the gratingposition to provide a frequency offset. Alternatively, a varying opticalpath length between the grating and the surface introduces the frequencyoffset. Alternatively, an asymmetric pattern for the gratingtransmission and appropriate signal manipulation/processing may be used.Alternatively, the direction may be determined by other means.

The method and device of the invention are applicable to a wide range ofapplications that require measurement of translation. One suchapplication is a “padless optical mouse”, that can effectively control acursor movement by moving the mouse across an optically diffuse surfacesuch as a paper or a desk-top. Another exemplary application for theinvention is for a “touch-point”, that translates finger movement over adevice aperture to control a cursor or any other translation or velocitycontrolled entity.

In accordance with a preferred embodiment of the invention, themeasurement apparatus comprises a light source for providing at leastpartially coherent radiation. The source radiation is directed toward anoptical one-dimensional or two-dimensional grating, which is preferablyclose to the reference surface. The light reflections from the gratingand from the surface interfere, and the light is collected through aspatial filter (for example, a lens and a pin-hole at its focal point)into a light-detector. The resulting interference signal contains beatsrelated to the relative translation of the optical apparatus and thesurface. Counting the “zero crossings” of the oscillating detectorsignal performs direct measurement of the amount of translation. Inpreferred embodiments of the invention, the translation is measureddirectly by counting zero crossings and is thus not subject to errorscaused by velocity changes. For preferred embodiments of the invention,substantially instantaneous position determination is established.

In many applications the translation direction as well as its magnitudeis required. In a preferred embodiment of the invention this isaccomplished by incorporating a phase shifting device (such as apiezoelectric transducer) which creates an asymmetric phase shiftpattern (typically a saw-tooth waveform) between the light reflectedfrom the grating and from the surface, enabling simple extraction of thedirection information. Alternatively, direction detection isaccomplished by using a preferably specially-designed asymmetrictransmission pattern for the grating/matrix (such as a saw-toothtransmission or other form as described herein) with appropriate signalprocessing/manipulation on the detector output signal. An asymmetrictransmission pattern provides means for motion direction detection inother velocimetry methods as well, such as speckle velocimetry.

A speckle-free, coherent detection of translation may be determined bycollecting the scattered light (the light which passes through thegrating and is reflected from the moving surface) with a spatial filter,such as a combination of a focusing lens and a pinhole aperture (orsingle mode optical fiber) at the focal position of the lens. The lightreflected from the surface is combined with a local oscillator lightfield (which is preferably the light reflected or diffracted by thegrating itself), which field is preferably a part of the light beam thatalso passes through the spatial filter. The interference with the stronglocal oscillator light source provides amplification of the detectedsignal by an intensity-sensitive photodetector. This coherent detectionmethod is termed homodyne detection.

The spatial filter is operative to spatially integrate light reflectedfrom the surface to a detector, such that the relative phases of thereflections from different locations on the surface are essentiallyunchanged when the surface moves with respect to the detector.Furthermore, the phase of a scatterer on the surface (as measured at thedetector) depends linearly on the surface translation. Also, the spatialfilter is ideally used to filter the local oscillator such that thedetector will integrate over no more than a single interference fringeresulting from the interference between the local oscillator and thelight reflected from the surface.

In one extreme case, the light incident on the surface is perfectlycollimated (i.e.—it is a plane wave). Thus, the spatial filter maysimply be a lens with a pinhole positioned at it's focal point. Anytranslation of the surface does not change the relative phases of thelight integrated by the spatial filter. The local oscillator formed bythe reflection or the diffraction from the reflector or grating is alsoperfectly collimated, so that it can also be passed through the spatialfilter (the spatial filter is positioned such that the image of thesource falls on or within the pin-hole). This forces a singleinterference fringe on the detector. No limitations are imposed (withregard to spatial filtering) on the spacing between the reflector andthe surface.

In another extreme case, the spacing between the surface and thereflector is negligible. This allows for the use of a substantiallynon-collimated incident beam while still maintaining the relative phasesof the reflections from the surface irrespective of it's translation andalso maintaining the same focusing point for the local oscillator andthe reflection from the surface. Optionally, the spatial filter may beimplemented with a lens and a pinhole positioned at the image plane ofthe reflection of the source as a local oscillator.

In order to have (at most) a single speckle integrated by the detector,the pinhole size should not exceed the size of about a single speckleformed by the reflection from the surface (for this reason, themeasurement may be termed “speckle-free”). Thus, if the detector itselfis small enough, it may serve as an integral part of the spatial filterand a pin-hole is not required.

The requirements of unchanged relative phases and single interferencefringe with the local oscillator at the detector can be fulfilled in amultitude of optically equivalent ways. In particular, the requirementmay be established using a single converging lens positioned before orafter the reflection of the light from the reflector. Alternatively, thelens and the reflector can be combined in a single optical device. Also,a collimating lens may be positioned between the beamsplitter and thesurface (i.e.—only light to and from the surface pass through thislens).

Non-ideal spatial filtering (as when the pin hole is too large, or whenit is out of focus for either the reflection from the surface or thelocal oscillator or both), results in deterioration of the signal andpossibly the addition of noise to the measurement. The level ofdeterioration depends on the amount and kind of deviation from theideal.

In a preferred method according to the present invention, both thesurface illumination and the reference light are provided using a singleoptical element, preferably a grating. The surface and reference lightshare a single optical path through all of the optical elements in thedevice. Moreover, the spatial amplitude and/or phase modulation, imposedon the light reaching the surface by the grating, provide additionalmeans for measuring the surface's translation. In particular, tangentialtranslation can be measured even for specular reflection from thegrating, where no Doppler shift exists, and identification of thedirection of motion can also be achieved.

There is thus provided, in accordance with a preferred embodiment of theinvention, a method for determining the relative motion of a surfacewith respect to a measurement device, comprising:

illuminating the surface with incident illumination having a coherencelength such that the illumination is reflected from portions of thesurface, wherein at least part of at least one of the incident andreflected illumination passes through a partially transmitting objectthat is part of the measuring device;

detecting the illumination reflected from the surface, to generate adetected signal; and

determining the relative motion of the surface parallel to the surface,from the detected signal,

wherein the object and the surface are situated within a distance thatis less than the coherence length of the detected illumination;

Preferably, the transmission of the object is spatially varying, morepreferably, periodic spatially varying.

Preferably, the object is partially reflecting, part of the incidentillumination is reflected or diffracted by the object, as a referenceillumination and detection of the illumination is coherent, utilizingsaid reference illumination.

There is further provided, in accordance with a preferred embodiment ofthe invention, a method for determining the relative motion of a surfacewith respect to a measurement device comprising:

placing a partially reflecting object, which is part of the measuringdevice, adjacent to the surface;

illuminating the object with incident illumination such that part of theincident illumination is reflected or diffracted by the object, as areference illumination and part is reflected from the surface;

coherently detecting the illumination reflected from the surfaceutilizing the reference illumination, to generate a detected signal; and

determining the relative motion of the surface parallel to the surface,from the detected signal.

Preferably, the object is a partially transmitting object and at leastpart of at least one of the incident and reflected illumination passesthrough the object.

Preferably, the reflection of the object is spatially varying,preferably periodically spatially varying.

In a preferred embodiment of the invention, the object is a grating.Preferably, the grating is placed sufficiently close to the surface suchthat the surface is in the near field of the grating. Alternatively, thegrating is placed sufficiently far from the surface such that thesurface is outside the near field of the grating.

In a preferred embodiment of the invention, the detected illumination isat least partly coherent.

There is further provided, in accordance with a preferred embodiment ofthe invention, a method for determining the relative motion of a surfacewith respect to a measurement device comprising:

placing an object comprising a grating, which is part of the measuringdevice, adjacent to the surface;

illuminating the grating with incident illumination such that at leastpart of the illumination is incident on and reflected from the surface,wherein at least one of the incident and reflected illumination passesthrough the grating;

detecting the illumination reflected from the surface;

generating a signal in response to the reflected illumination; and

determining the relative motion of the surface parallel to the surface,from the detected signal,

wherein the surface is in the near field of the grating.

In a preferred embodiment of the invention, the illumination reflectedfrom the surface is frequency shifted from that of the illuminationreflected from or diffracted by the object and wherein determining themotion comprises determining the motion based on the frequency shift.

In a preferred embodiment of the invention, determining the motioncomprises determining variations in the amplitude of the signal withposition. Preferably, the motion is determined from zero crossings ofthe detected signal.

In a preferred embodiment of the invention, the object has atransmission characteristic that is spatially non-symmetric and themethod includes determining the direction of the relative motion basedon the detected signal.

In a preferred embodiment of the invention, the method includesdetermining the magnitude and direction of the translation utilizing twodetectors which produce different detected signals depending on thedirection of the translation. Preferably, the method includesdetermining the direction of translation from the sign of a phasedifference between the different detected signals.

In a preferred embodiment of the invention, the illumination isperpendicularly incident on the surface.

In a preferred embodiment of the invention, the surface is an opticallydiffusely reflecting surface.

In a preferred embodiment of the invention, the surface has no markingsindicating position.

In a preferred embodiment of the invention, the illumination comprisesvisible illumination. Alternatively or additionally, the illuminationcomprises infra-red illumination.

Preferably, the method includes determining motion in two dimensionstransverse to the surface. Preferably, the method includes determiningmotion in a direction perpendicular to the surface.

There is further provided, in accordance with a preferred embodiment ofthe invention, a scanner for reading a document by movement of thescanner over the document comprising:

an optical reading head which detects patterns on the surface of thedocument; and

an optical detector which determines the motion of the scanner as it istranslated across the surface of the document, wherein the opticaldetector utilizes the method of the invention to determine thetranslation.

There is further provided, in accordance with a preferred embodiment ofthe invention, an optical mouse comprising:

a housing having an aperture facing a surface; and

an optical motion detector which views the surface through the aperture,wherein the optical motion detector utilizes the method of the inventionto determine the translation of the housing with respect to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from the followingdescription of the preferred embodiments of the invention read inconjunction with the attached drawings in which:

FIG. 1 is a schematic representation of a preferred embodiment of amotion transducer, in accordance with a preferred embodiment of theinvention;

FIG. 2 is a graph of a grating transmission function, in accordance witha preferred embodiment of the invention;

FIGS. 3A, 3B and 3C are schematic representations of preferredembodiments of integrated motion transducers, in accordance withpreferred embodiments of the invention;

FIG. 4 is a schematic diagram of an optical mouse in accordance with apreferred embodiment of the invention;

FIGS. 5A and 5B are schematic diagrams of a mouse/finger translationmeasurement device, in accordance with a preferred embodiment of theinvention;

FIG. 6 is a schematic diagram of a scanning pen in accordance with apreferred embodiment of the invention;

FIG. 7 is a diagram of a rotary encoder, in accordance with a preferredembodiment of the invention;

FIG. 8 is a schematic diagram of a fiber-optic-based translationmeasurement device, in accordance with a preferred embodiment of theinvention; and

FIG. 9 is a simplified and generalized block diagram of electroniccircuitry, suitable for use in preferred embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows apparatus 10 for the measurement of the translation of asurface 12, in accordance with a preferred embodiment of the invention.Apparatus 10 comprises a source of at least partially coherent,preferably collimated optical radiation 14, such as a laser. Preferably,the laser is a diode laser, for example a low power infra-red laser.While other wavelengths can be used, an infra-red laser is preferredsince it results in eye-safe operation. The source is preferablycollimated. However, a non-collimated source may be used if compensationas described below is used. While it is desirable to use a collimatedsource from depth of field considerations, the collimation need not beparticularly good.

Apparatus 10 also includes a one-dimensional or two dimensionalreflective grating 16 which is closely spaced from surface 12. Thelimitations as to spacing of grating 16 from surface 12 are describedbelow. Light which is reflected from (or diffracted by) grating 16 andlight reflected by surface 12 are both incident on a spatial filter(composed of a lens 18 and a pinhole 20) before being detected by anoptical detector 22. The resulting interference gives rise to a beatsignal that depends on the motion of the surface. It should be notedthat this and other Figs. show an exaggerated spacing between grating 16and surface 12, for clarity. As indicated from FIG. 1, radiation isreflected from the surface in substantially all directions. Thisradiation is shown only in FIG. 1 and not in the other drawings forclarity of presentation.

In FIG. 1 the light is seen as being incident on the surface from anangle; however, it is possible for the light to be incident at thenormal to grating 16. Moreover, while FIG. 1 shows the incident lightangle equal to the detection angle, such that light reflected from thegrating (or zeroth order diffraction) is used for the local oscillator,first or higher order diffraction by the grating can be effectivelyused. Zero order has the advantages that it is wavelength independent(stability of the wavelength is not important). The incident light canbe pulsed or continuous. In FIG. 1, light diffracted at the −1 and +1orders are indicated by reference numbers 19 and 21 respectively. Lightwhich is scattered by the surface is indicated by reference number 17.

In the preferred embodiment of the invention shown in FIG. 1,speckle-free, coherent detection (homodyne or heterodyne, homodyne shownin FIG. 1) is used to determine tangential motion. Such detectionresults in an intrinsic amplification of the signal used for measurementresulting in a high dynamic range.

The reference local oscillator field for the coherent detection isprovided by reflections from grating 16, placed close to the movingsurface. The interference of the reflections from the grating and themoving surface on the detector give rise to a translation dependentoscillating signal. The incorporation of a near-surface reflection froma grating as the origin of the local oscillator field may give multipleadvantages, including at least some of the following:

1. The grating is a single element that combines the roles of a beamsplitter and a mirror in a coherent homodyne/heterodyne detectionoptical setup, thus making the optical system simple, robust and withfew alignment requirements.

2. The grating causes spatially periodic intensity and/or phasemodulation of the illumination reflected from the surface, if thesurface is placed within the near field of the grating. This enablesdetection of translation using the specular (zeroth order) reflection asthe reference wave.

3. High order reflections (±1^(st), ±2_(nd), etc.) serve as localoscillator fields for high resolution detection of the surface'stranslation. A translation dependent phase shift between the referenceand surface waves at non-specular reflection orders produce oscillationsrepresentative of the translation.

4. Translation detection can be frequency biased by periodic shifting ofthe grating position (e.g. sawtooth modulation), enabling determinationof direction as well as magnitude of the translation.

5. A two-dimensional grating provides reference (local oscillator) wavesand modulation of the illumination of the surface and reflections fromit for two orthogonal translation directions in a single element, for afull two-dimensional transverse motion measurement coverage.

6. Measurement at different grating orders provides different componentsof the translation or velocity vector of the surface. For specularreflection, for example, translation along the axis perpendicular to thegrating can be measured independently of translation in the otherdirections. This allows for three dimensional translation measurement byadding the measurement or the calculation of the axial component for twomeasurements performed for the same transverse axis but in differentgrating orders.

7. Asymmetric grating transmission functions (amplitude and/or phase)enable direction detection in all reflected orders, using appropriatesignal manipulation/analysis.

8. Frequency biasing using local oscillator phase shifting, incombination with the amplitude modulation resulting from the grating atnear field provide for simultaneous measurement of 2-D translation (in atransverse and axial translation plane) by a single detector.

In addition to spatial filtering related restrictions, the alloweddistance between the grating and the surface generally depends on thegrating period Λ, the light wavelength λ, the spectral coherence widthΔλ, the illuminated area and the incident and reflected beam angles.

For those preferred embodiments of the invention which utilize the lightreflected or diffracted from the grating as a local oscillator, it ismost preferable for the spacing between the surface 12 and the grating16 to be smaller than the coherence length of the light, given by≈λ²/Δλ, where Δλ is the spectral width of radiation reaching thedetector, and not necessarily the spectral width of the light source.Thus, by proper spectral filtering along the optical path, the spectralcontent reaching the detector can be limited and its coherence lengthincreased, if this is necessary.

For those preferred embodiments of the invention, in which the modulatedtransmission pattern plays a major role in the detection scheme, thespacing between the grating and surface 12 should also be within thenear-field distance from the grating, ≈Λ²/4λ. For the followingembodiments the spacing is assumed to be near field. This requirement isrelaxed for the cases where it is not essential.

Relative motion of the surface can be measured in a number of ways.Consider the incident field and the grating field transmission function,respectively:

E(t)=E ₀cos(ω₀ t)  (1)

$\begin{matrix}{{A(x)} = {\sum\limits_{m}{c_{m}{\cos \left( {{2\quad \pi \quad {{mx}/\Lambda}} + \psi_{m}} \right)}}}} & (2)\end{matrix}$

The grating is assumed to be a pure amplitude grating with period Λ, sothat its transmission is the sum over non-negative spatial frequencieswith real coefficients. A similar formalism applies also to binary phasegrating, or some general phase gratings, which can also be used in thepractice of the present invention. For the general case of bothamplitude and phase gratings a phase retardation term is added. Forsimplicity of the description the following description is based on apure amplitude grating. However, it should be understood that othergratings can be utilized. Unimportant constant factors are also omittedin various parts of the following mathematical treatment.

Plane-wave illumination by the light source over the grating area isassumed (i.e.—a collimated beam), but is not strictly necessaryprovided, for example, the non-collimation is compensated in anotherpart of the system (e.g.—the spatial filter). It is assumed forsimplicity that the incident light is perpendicular to the grating (andnot as shown in FIG. 1). Oblique incident light (in the direction of thegrating lines and/or perpendicular to it) gives substantially the sameresults, with shifted reflection angles. Thus, the grating fieldcontains a series of reflected diffraction orders, arrangedsymmetrically about the specular reflection component (zeroth order) andobeying the angular condition (for the n-th order):

sin(α)=nλ/Λ  (3)

As shown in FIG. 1, a spatial filter in front of the detector ispreferably comprised of focusing lens 18 and narrow pinhole 20 at thefocal point of the lens. Such a spatial filter is preferably adjusted toselect only a single spatial frequency component to reach the detector.The pinhole can be replaced by a single-mode optical fiber, having asimilar core diameter and leading the light to a remote detector. Thespatial filter is aligned such that one of the diffraction ordersreaches the detector, and serves as the local oscillator for homodynedetection of the reflected radiation, or for heterodyne detection asdescribed below. The local oscillator field is given by:

E _(LO)(t)=E _(n) cs(ω₀ t+φ _(n))  (4)

The reflected field from the moving surface in the same direction as then-th diffraction order is represented by an integral over theilluminated surface area of independent reflections from the surface.Integrating over the direction parallel to the grating lines (y) andover the direction normal to the surface (corresponding to lightpenetration into the surface), results in a reflected field equal to:$\begin{matrix}\begin{matrix}{{E_{r}(t)} = \quad {E_{0}{\int_{x_{1}}^{x_{2}}{{{{xA}(x)}}{r\left( {x - {p(t)}} \right)}\cos\left( {{\omega_{0}t} + {2\quad \pi \quad {{nx}/\Lambda}} +} \right.}}}} \\{\quad \left. {\varphi \left( {x - {p(t)}} \right)} \right)}\end{matrix} & (5)\end{matrix}$

where r(x) and φ(x) are location dependent amplitude and phasereflectance of the surface, respectively. The reflectance is assumed tobe time-independent during the measurement, with both r and φ beingrandom variables of the position x. The translation of the surface fromits initial position is given by p(t), with p(0)=0. The periodic phaseterm 2πnx/Λ arises from the reflection at an angle sin(α)=nλ/Λ. Theintegration limits are from x₁ to x₂, both determined by the illuminatedarea.

Changing the integration variable from x to x-p(t), corresponding to thesymmetric situation of a static surface and moving grating with respectto the reference coordinate system: $\begin{matrix}\begin{matrix}{{E_{r}(t)} = \quad {E_{0}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{{{xA}\left( {x + {p(t)}} \right)}}{r(x)}}}}} \\{\quad {\cos\left( {{\omega_{0}t} + {2\quad \pi \quad {{{np}(t)}/\Lambda}} + {2\quad \pi \quad {{nx}/\Lambda}} + \left. {\varphi (x)} \right)} \right.}}\end{matrix} & (6)\end{matrix}$

with integration limits now extending from x₁-p(t) to x₂-p(t) and thusbeing time-dependent.

Replacing A(x) with its Fourier series and writing φ_(n)(x)=φ(x)+2πnx/Λgives: $\begin{matrix}\begin{matrix}{{E_{r}(t)} = \quad {E_{0}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}{\sum\limits_{m}{c_{m}\cos \quad \left( {{2\quad \pi \quad {{mx}/\Lambda}} + {2\quad \pi \quad {{{mp}(t)}/\Lambda}} + \psi_{m}} \right)}}}}}} \\{\quad {{r(x)}{\cos \left( {{\omega_{0}t} + {2\quad \pi \quad {{{np}(t)}/\Lambda}} + {\varphi_{n}(x)}} \right)}}}\end{matrix} & (7)\end{matrix}$

The (optical) phase of a scatterer on the surface linearly depends onthe translation p(t), with φ=φ_(n)(x)+2πnp(t)/Λ. For specular reflection(n=0), the phase is a constant.

Both the reflected field and the local oscillator field reach thedetector. Since the detector measures intensity, which is proportionalto the square of the field, the intensity is given by:

I(t)=(E _(LO)(t)+E _(r)(t))² =E _(LO)(t)²+2E _(LO)(t)E _(r)(t)+E_(r)(t)²  (8)

Assuming that the local oscillator field is much larger than thereflected field, E_(LO)>>E_(r) and that the detector integration time ismuch longer than an optical period time but much shorter than Λ/nVmax(where V_(max) is the maximum measured velocity), integration overoptical frequencies gives just a DC component while other variations aredetected instantaneously. Under these assumptions, the first intensityterm is replaced by a constant I_(LO)=E_(LO) ²/2 and the third intensityterm is neglected, i.e., I_(r)=E_(r) ²/2=0. In this preferred embodimentof the invention, the ratio of the strength of the local oscillatorfield and of the reflected field is intrinsically large, since thereflection from the grating is directed only to specific narrow ordersand the reflection from the diffuse surface is scattered over a broadangle.

Although the third term is generally neglected in the followingdiscussion, translation measurement utilizing the spatial transmissionmodulation is possible even if the third term alone is present, i.e.,when the light reflected from the surface is not combined with areference reflected or diffracted from the grating. This may be achieved(if desired) by selecting an angle which lies between grating orders. Itdoes have the advantage of significantly relaxed alignment constrains(it is only required to be in the focal plane of the spatial filter),but will generally be less accurate and with a low signal-to-noiseratio.

The local oscillator field serves as a very strong amplifier in thefirst stage of signal detection. In this respect it is stronglypreferred to keep the local oscillator field as noise-free as possible,since its noise transfers to the detected signal directly.

The measured cross term is equal to:

I _(s)(t)=E _(n)cos(ω₀ t+ω _(n))E _(r)(t)  (9)

Inserting the oscillating field term cos(ω₀t) into the integral forE_(r)(t) and using the cosine sum relationshipcosαcosβ=0.5(cos(α+β)+cos(α−β)) for the right-most cosine in (7),results in one intensity component at twice the optical frequency (2ω₀)and another with slowly varying phase. The fast oscillating componentaverages to zero because of the detector's time response. The remainingsignal is: $\begin{matrix}\begin{matrix}{{I_{s}(t)} = \quad {I_{n}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}{\sum\limits_{m}{c_{m}\cos \quad \left( {{2\quad \pi \quad {{mx}/\Lambda}} + {2\quad \pi \quad {{{mp}(t)}/\Lambda}} + \psi_{m}} \right)}}}}}} \\{\quad {{r(x)}{\cos \left( {{2\quad \pi \quad {{{np}(t)}/\Lambda}} + {\varphi_{n}(x)} - \phi_{n}} \right)}}}\end{matrix} & (10)\end{matrix}$

Exchanging summation with integration, the contribution of each term tothe sum is: $\begin{matrix}\begin{matrix}{{I_{s,m}(t)} = \quad {I_{n}c_{m}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad \cos \quad \left( {{2\quad \pi \quad {{mx}/\Lambda}} + {2\quad \pi \quad {{{mp}(t)}/\Lambda}} + \psi_{m}} \right)}}}} \\{\quad {{r(x)}{\cos \left( {{2\quad \pi \quad {{{np}(t)}/\Lambda}} + {\varphi_{n}(x)} - \phi_{n}} \right)}}}\end{matrix} & (11)\end{matrix}$

The grating has a period Λ. The average transmission of the grating isgiven by the m=0 term in the expansion. Consider the requirement thatthe ‘zero average’ grating function (function minus the zero order term)has only two zero crossings in any interval of length Λ. Thisrequirement is equivalent to having c₁>>{c_(m), m>1 }. This lastrequirement enables us to concentrate on just two terms in the sum overgrating harmonics, the m=0 and m=1 terms. For these two terms we canwrite: $\begin{matrix}{{I_{s,0}(t)} = {I_{n}c_{0}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad {r(x)}{\cos \left( {{2\quad \pi \quad {{{np}(t)}/\Lambda}} + {\varphi_{n}(x)} - \phi_{n}} \right)}}}}} & (12) \\\begin{matrix}{{I_{s,1}(t)} = \quad {I_{n}c_{1}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad \cos \quad \left( {{2\quad \pi \quad {x/\Lambda}} + {2\quad \pi \quad {{p(t)}/\Lambda}} + \psi_{1}} \right)}}}} \\{\quad {{r(x)}{\cos \left( {{2\quad \pi \quad {{{np}(t)}/\Lambda}} + {\varphi_{n}(x)} - \phi_{n}} \right)}}}\end{matrix} & (13)\end{matrix}$

Attention is now focused on specific diffraction orders in the reflectedand diffracted waves from the grating, the n=0 (specular reflection) andn=±1 directions.

For the specular reflection term, the m=0 contribution is:$\begin{matrix}{{I_{s,0}(t)} = {I_{0}c_{0}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad {r(x)}{\cos \left( {\varphi (x)} \right)}}}}} & (14)\end{matrix}$

For a diffuse surface with constant brightness this term will be nearlyconstant, and will change slowly as and when the average reflection fromthe surface changes. The m=1 term is: $\begin{matrix}\begin{matrix}{{I_{s,1}(t)} = \quad {I_{0}c_{1}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad \cos \quad \left( {{2\quad \pi \quad {x/\Lambda}} + {2\quad \pi \quad {{p(t)}/\Lambda}} + \psi_{1}} \right)}}}} \\{\quad {{r(x)}{\cos \left( {\varphi (x)} \right)}}\quad} \\{= \quad {\cos \left( {2\quad \pi \quad {{p(t)}/\Lambda}} \right)I_{0}c_{1}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{x}}}} \\{= \quad {{{\cos \left( {{2\quad \pi \quad {x/\Lambda}} + \psi_{1}} \right)}{r(x)}\quad {\cos \left( {\varphi (x)} \right)}} - {{\sin \left( {2\quad \pi \quad {{p(t)}/\Lambda}} \right)}I_{0}c_{1}}}} \\{\quad {\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad {\sin \left( {{2\quad \pi \quad {x/\Lambda}} + \psi_{1}} \right)}{r(x)}{\cos \left( {\varphi (x)} \right)}}}} \\{\equiv \quad {{{\cos \left( {2\quad \pi \quad {{p(t)}/\Lambda}} \right)}{I_{c}(t)}} + {{\sin \left( {2\quad \pi \quad {{p(t)}/\Lambda}} \right)}{I_{s}(t)}}}} \\{\equiv \quad {{I(t)}{\cos \left( {{2\quad \pi \quad {{p(t)}/\Lambda}} + {\vartheta (t)}} \right)}}}\end{matrix} & (15)\end{matrix}$

where the intensity I(t) and phase θ(t) result from integrals overrandom variables corresponding to the amplitude and phase reflection ofthe diffuse surface at a spatial frequency 1/Λ. For diffuse surfaceswith single reflectors larger than the spatial wavelength Λ thecontribution will come from grain boundaries, while for diffuse surfaceshaving small particle sizes there will be strong contributions for allspatial frequencies up to 1/d, where d is the average particle size.

The rate of change of these “random walk” variables depends on theaverage time it takes a given set of reflection centers {x_(i)} to bereplaced by a new set, which in turn is related to the change of theintegration region above, τ æ (x₁-x2)/v=L/v, where v is theinstantaneous velocity and L is the illuminated size of the grating. Ifa large number of grating periods are illuminated such that L>>Λ, theresult is fast oscillations with a slowly varying statistical amplitudeand phase. The error of the translation measurement is proportional toΛ/L and is independent of the velocity.

In summary, for specular reflection translation measurement:

1. The measured signal at the detector output oscillates at a frequencyof v/Λ. Detection and counting of the zero crossing points of thissignal gives a direct translation measurement, each zero crossingcorresponding to a Δp=Λ/2 translation, provided that the translationdirection does not change during the measurement.

2. The measured signal's amplitude and phase are slowly varyingstatistical ensemble sums. The relative accuracy of the measurement isproportional to Λ/L, L being the illuminated grating size.

3. The spacing between the surface and the grating should preferably besmaller than both the near field distance, ≈Λ²/4λ, and the coherencelength of the light reaching the detector, ≈λ²/Δλ.

The first order reflection, unlike the specular reflection, carries alsoa Doppler phase shift. Looking again at the contribution of the m=0, 1spatial frequency components gives: $\begin{matrix}{{I_{s,0}(t)} = {I_{1}c_{0}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad r\quad (x){\cos \left( {{2\quad \pi \quad {{p(t)}/\Lambda}} - {\varphi_{1}(x)} - \phi_{1}} \right)}}}}} & (16) \\\begin{matrix}{{I_{s,1}(t)} = \quad {I_{1}c_{1}{\int_{x_{1} - {p{(t)}}}^{x_{2} - {p{(t)}}}{{x}\quad \cos \quad \left( {{2\quad \pi \quad {x/\Lambda}} + {2\quad \pi \quad {{p(t)}/\Lambda}} + \psi_{1}} \right)}}}} \\{\quad {{r(x)}{\cos \left( {{2\quad \pi \quad {{p(t)}/\Lambda}} + {\varphi_{1}(x)} - \phi_{1}} \right)}}}\end{matrix} & (17)\end{matrix}$

Using a decomposition of the cosine term in (16) as in (15) results in:

I _(s),0(t)=I ₀(t)cos(2πp(t)/Λ+Φ₀(t))  (18)

In a similar manner the expression for the m=1 term, (17) is:

I _(s),1(t)=I ₁(t)cos(4πp(t)/Λ+Φ₁(t))  (19)

Equation (19) neglects a slowly varying term that adds to the averagedetector signal (the “DC” component). An analysis of equations (16)-(19)shows that if c₀>>c₁, the zero crossings of the signal correspond toΔp=Λ/2, while if c₀<<c₁, zero crossings correspond to Δp=Λ/4. Thisresult can be expanded to other reflection orders n>1, where, if c₀>>c₁the measured signal will oscillate according to np(t)/Λ. For |n|>1, thec₁ term amount to oscillations in two side bands around the c₀oscillations as in amplitude modulation of a higher frequency signal.Notice that the m=0 term does not require near field conditions, so byfixing the distance to the moving surface so it is larger than the nearfield limit ≈Λ²/4λ but smaller than the coherence length ≈λ²/Δλ, the m=0contribution is dominant. Alternatively, a transmission function for thegrating such that c₀>>c₁ even in the near field can be used.

The frequency associated with the c₀ oscillations depends on thetransverse as well as axial (perpendicular) translation component (notshown in the above mathematical development). Conversely, the amplitudemodulation (through the c₁ component) depends solely on the transversecomponent. When the frequency of the c₀ oscillations is sufficientlyhigh, this frequency can be measured by the frequency-related techniquedescribed above, simultaneously with the detection of the amplitudemodulation frequency to measure the transverse translation component. Inthis way, 2-D translation measurement (including motion perpendicular tothe plane of the surface—i.e., axial translation) may be achieved usinga single detector.

By frequency biasing the reference signal, the ratio between the carrierfrequency and the amplitude modulation frequency can be made large,improving the measurement accuracy as well as allowing for detection ofthe direction of translation. Also, using specular reflection from thegrating as a local oscillator enables a clear distinction to be madebetween the transverse translation component (indicated by the amplitudemodulation) and the axial translation component (indicated by the phaseor frequency shift of the carrier frequency).

Furthermore, the phase shifting may be combined with an asymmetrictransmission pattern of the grating (e.g.—sawtooth pattern) for thepurpose of transverse translation direction detection. Alternatively,the grating may be displaced for direction detection in the twodimensions.

In essence, for the non-specular diffraction embodiments of theinvention, two quasi-plane waves are selected for detection by thedetector. One of these waves is the result of the nth order diffractionfrom the grating. The second plane wave is generated by the selection ofone plane wave (by the spatial filter) from the reflections from thesurface.

In summary for translation measurement using non-specular diffraction(and assuming constant velocity for clarity of the discussion):

1. The measured signal at the detector output oscillates at a frequencyof nv/Λ, where n is the order number. Detection and counting of the zerocrossing points of this signal gives a direct translation measurement,each zero crossing corresponding to Λ/2n translation provided that thetranslation direction is not switched during the measurement.

2. The measured signal's amplitude and phase are slowly varyingstatistical ensemble sums. The relative accuracy of the measurement isproportional to Λ/nL, L being the illuminated grating size.

3. The distance between the surface and the grating should preferably besmaller than the coherence length of the light reaching the detector,≈λ²/Δλ.

Even though the absolute time-varying translation |p(t)| can be measuredvery accurately its direction is preferably determined using one of themethods described below.

In one preferred embodiment of the invention, direction may bedetermined by applying an additional phase shift between the reference(local oscillator) field and the reflected field. This additional phaseshift can be manifested, for example, by moving the grating towards oraway from the surface. This movement does not change the phase of thefield incident upon the surface, so that the reflected field isidentical to that given above. The local oscillator field, however,acquires an additional phase shift due to this translation that dependson the grating displacement d(t).

Keeping the distance between the grating and the surface almost constantand introducing a fixed frequency shift between the reflected and localoscillator fields can be achieved by making d(t) a periodic saw-toothfunction: $\begin{matrix}{{{d_{n}(t)} = {D_{n}{\int_{0}^{t}{\left\lbrack {\tau^{- 1} - {\sum\limits_{k = 0}^{\infty}{\delta \left( {t^{\prime} - {k\quad \tau}} \right)}}} \right\rbrack {t^{\prime}}}}}}{D_{n} = \frac{\lambda}{1 + \sqrt{1 - \left( \frac{n\quad \lambda}{\Lambda} \right)^{2}}}}} & (20)\end{matrix}$

with τ as the cycle time for the saw-tooth, fixing the amplitude of thesaw-tooth to give 2π phase shift (or multiples of 2π) for reflection atthe nth diffraction order. The frequency shift due to this motion isτ⁻¹, and if τ⁻¹>nv/Λ is maintained, the direction of the motion isdetermined without ambiguity according to the frequency of oscillationof the detector signal, namely τ⁻¹+nv/Λ. Alternatively, the translation(both positive and negative) is determined directly by counting the zerocrossing in the detected signal and subtracting it from the result of asimultaneous count of the oscillator frequency τ⁻¹.

If the saw tooth amplitude is not ideal, (i.e., it does not correspondto integer multiples of the wavelength) the direction can still bedetermined, however, the formulation is more complicated. As usedherein, the term “saw-tooth” includes such non-ideal variations.

An alternative way of introducing a periodic phase shift between thelocal oscillator field and the field reflected from the surface is tomodulate the optical path length between the grating and the surface.This is preferably achieved by a transparent piezo-electric elementmounted between the grating and the surface.

An alternative methodology to break the symmetry between positive andnegative relative translation, so that the translation direction can bedetected, is to use an asymmetric function for the transmission(amplitude and/or phase) function of the grating. For simplicity, theformalism is developed for an amplitude grating. For simplicity, assumethat the grating is large compared to the line spacing along thetranslation axis and that k point scatterers are illuminated through thegrating. Scatterers entering or leaving the illuminated area areneglected (this will appear as a noise factor in a comprehensivetreatment). After the interference with the local oscillator (which isnot shifted here) and filtering the optical frequencies, the resultingsignal can be written as: $\begin{matrix}{{I_{s}(t)} = {I_{n}{\sum\limits_{i = 1}^{k}{r_{i}{A\left( {x_{i} + {p(t)}} \right)}{\cos \left( {{2\quad \pi \quad {{{np}(t)}/\Lambda}} + \varphi_{i}} \right)}}}}} & (21)\end{matrix}$

where r_(i), x_(i) and Φ_(i) are the reflectance, the position (at timet=0) and the relative phase (with respect to the local oscillator),respectively, of a scatterer i. For a diffuse body these are all randomvariables. This presentation of the detector signal is used for thefollowing direction-detection mechanisms.

For specular reflection: $\begin{matrix}{{I_{s}(t)} = {I_{0}{\sum\limits_{i = 1}^{k}{r_{i}{A\left( {x_{i} + {p(t)}} \right)}{\cos \left( \varphi_{i} \right)}}}}} & (22)\end{matrix}$

Assuming that p(t)=vt. i.e.—changes in the surface velocity arerelatively small during the integration time used for determination ofthe translation direction. Thus, the first and second derivatives of thereceived signal are: $\begin{matrix}{{I_{s}^{\prime}(t)} = {I_{0}v{\sum\limits_{i = 1}^{k}{r_{i}{\cos \left( \varphi_{i} \right)}\frac{}{x}\left( {A\left( {x_{i} + {vt}} \right)} \right)}}}} & (23) \\{{I_{s}^{''}(t)} = {I_{0}v^{2}{\sum\limits_{i = 1}^{k}{r_{i}{\cos \left( \varphi_{i} \right)}\frac{^{2}}{x^{2}}\left( {A\left( {x_{i} + {vt}} \right)} \right)}}}} & (24)\end{matrix}$

Assume that A(x) is constructed such that$\frac{^{2}{A(x)}}{x^{2}} = {\eta \quad {\frac{{A(x)}}{x}.}}$

In this special case it is evident that: I_(s) ^(″)(t)=ηv.I_(s) ^(′)(t).Thus, the magnitude, and more importantly, the sign of the translationvelocity (i.e.—the translation direction) can be derived from the ratiobetween the first and second time-derivatives of the detector signal.

If the velocity cannot be assumed to be constant during thedirection-decision integration time, then the derivatives may beperformed with respect to the measured translation (which is known fromthe zero-crossing or from another detector with higher accuracy operatedin parallel). If only the direction is required (and not the velocitymagnitude), it is sufficient to check if the first and secondderivatives carry the same sign (one direction) or not (oppositedirection). A simple XOR (exclusive OR) operation after sign-detectionof the derivatives will be “1” if the sign of η is opposite to the signof v and “0” if they are the same.

An example of A(x) that satisfies the constant derivative ratio is acombination of exponents like: $\begin{matrix}{{A(x)} = \left\{ \begin{matrix}{{A\left( {1 - ^{{- {\gamma {({x - {j\quad \Lambda}})}}}/\Lambda}} \right)}:} & {{{if}\quad j\quad \Lambda} \leq x < {\Lambda \left( {j + {1/2}} \right)}} \\{{A\left( {^{{- {\gamma {({x - {{({j + {1/2}})}\Lambda}})}}}/\Lambda} - ^{{- \gamma}/2}} \right)}:} & {{{if}\quad {\Lambda \left( {j + {1/2}} \right)}} \leq x < {\Lambda \left( {j + 1} \right)}}\end{matrix} \right.} & (25)\end{matrix}$

where the pattern is repetitive with a cycle Λ. It is evident that forthis pattern the first and second (and in fact all) derivatives have aconstant ratio as required, of η=−γ/Λ. But, the singularity points inmultiples of Λ/2 introduce “noise” to the measurement. Thesesingularities increase the error probability as the number of scatterersgrow, since each one will appear in the received signal when a scattererpasses across it. The relative noise contribution is reduced as thedirection detection integration time increases.

The pattern is assumed to be the intensity of illumination on thesurface. Thus, the requirement for the near field is more stringent thanthe similar requirement for measuring translation magnitude alone in n=0specular reflection. An assumed transmission pattern is shown in FIG. 2,for γ=5. This can be achieved by having a partiallyreflecting/transmitting property for the grating, having an amplitudetransmission function such as that shown in FIG. 2.

A relaxed requirement from the transmission pattern is that thederivatives will have a constant sign relationship (i.e.—they are notexactly proportional, but their ratio's sign is constant along thepattern). Here, direction-detection is still assured for a singlescatterer, but the error probability is higher than in the former caseas the number of scatterers gets larger (even without the effect of thesingularities).

A similar analysis is possible for high-order reflection (|n|>>1).Again, for simplicity the surface is assumed to move with a constantvelocity, v. Equation (21) can be looked at as a sum ofamplitude-modulated signals of a carrier with frequency nv/Λ.

A(x) is now assumed to be asymmetric (e.g.—sawtooth waveform). For|n|>>1, the detector's signal envelope matches the transmission functionfor translation in the “positive” direction and is the inverse image theother way. Thus, if the number of scatterers is small (the limit beingdependent on the grating order n), the translation direction isrepresented by the sign of the first derivative of the detected signal'senvelope. In addition, the magnitude of the envelope derivative isproportional to the magnitude of the translation velocity.

An asymmetric transmission pattern enables direction detection forspeckle velocimetry. The detector signal resulting from a random specklepattern, filtered by a grating with intensity transmission pattern A(x)adjacent to the detector, can be represented as: $\begin{matrix}{{I_{s}(t)} = {I_{0}{\sum\limits_{i = 1}^{k}{r_{i}{A\left( {x_{i} + {p(t)}} \right)}}}}} & (26)\end{matrix}$

where r_(i) and x_(i) are the intensity and position of the i-th“speckle”, respectively, and p(t) the surface translation. Assumingconstant velocity, p(t)=vt, the detector signal time derivative is:$\begin{matrix}{{I_{s}^{\prime}(t)} = {I_{0}v{\sum\limits_{i = 1}^{k}{r_{i}\quad \frac{}{x}\quad \left( {A\left( {x_{i} + {vt}} \right)} \right)}}}} & (27)\end{matrix}$

The intensities r_(i) are positive values. Thus, if dA/dx is constant,then the derivative of the detector signal is indicative of thetranslation direction. Such a pattern is accomplished using sawtoothtransmission pattern. The discontinuities in the pattern add noise tothe measurement, requiring the use of an appropriate integrationinterval in order to limit the error probability. The motion velocity isdetermined from the frequency of oscillations of the detector signal.

Of course, it is possible to utilize mechanical or other means (e.g.—anaccelerometer) to determine the direction of motion as a complementarycomponent in an OTM device.

As was noted above, fluctuations in the source amplitude are directlytransferred to the received signal via the local oscillator field. Inorder to minimize such noise, in accordance with a preferred embodimentof the invention, a signal proportional to the source amplitude isdetected and the resulting signal (termed the “compensation” detectorand signal) is subtracted from the detector signal. This detection canbe performed, for example, by:

Splitting the source beam with a beamsplitter (which need not beaccurately aligned) and directing the diverted beam to the compensationdetector, or

Directing any of the light beams reflected from the grating to acompensation detector without spatial-filtering it (but potentially withconsiderable attenuation). Conveniently this may be one of the gratingorders not used for the spatial filter measurement. e.g.—use order 1 forspatial filter and order 0 for source-noise compensation.

The output of the compensation detector is amplified (or attenuated) sothat the resulting difference signal is as close to zero as possiblewhen the surface is not moving relative to the device (or when the“window” is closed with an opaque cover), thus compensating for the E₀ ²factor.

In order to compensate for the E₀ multiplier of the E_(r) component, thesignal from the compensation detector may be the control voltage of again-controlled amplifier in one of the stages of the signalamplification (after the first compensation by subtracting the E₀ ²component). The gain should be approximately proportional to the inverseof the square-root of the compensation signal.

FIG. 3A shows a preferred implementation of a translation detector, inaccordance with a preferred embodiment of the invention, in which zerothorder detection is used and which does not incorporate directiondetection, or in which the detection of the direction is based on anasymmetric grating transmission pattern and appropriate signal analysis.FIG. 3A shows an integrated optical chip translation device 30 which issuitable for mass production. It utilizes only a few components that canbe manufactured in large quantities for a low price. Device 30 comprisesa laser diode 32, preferably a single transverse mode laser. Laser lightfrom laser diode 32 is preferably collimated by a lens 34, which ispreferably a diffractive collimating lens, etched into or deposited ontothe surface of an optical chip substrate 36 of glass, quartz or thelike, preferably coated with non-reflective layers on both sides otherthan in designated areas. A grating 38 preferably, either an amplitudeor phase type grating is mounted on optical chip substrate 36. Grating38 is preferably etched or deposited onto the lower surface of substrate36. Light reflected by the grating and light reflected from a surface 42is reflected by two reflective surfaces 40 and 41 and focused by a lens44, preferably a reflective diffractive focusing lens, etched into thesurface of optical chip substrate 36. After further reflection by areflective surface 45, a pinhole 46, formed in a reflective/opaque layerformed at the focus of lens 44, passes only a plane wave from surface 42and the reflected light from grating 38 to a detector 50, for example aPIN photo diode or similar device. A compensating detector 52 ispreferably placed behind lens 44 detects a portion of the lightreflected by grating 38. A controller 54, comprising a laser diodedriver/modulator for activating laser diode 32, detection amplifiers andzero crossing counting circuits or frequency detection means used fordetermining the translation velocity and translation of the surface.Compensation detector supplies a compensation signal proportional to theamplitude of the local oscillator for reducing any residual effects ofvariations in the laser output. For reduction of noise, twisted wirepairs, shielded wires or coaxial cable are preferably used to carrysignals to and from controller 54. Preferably, the apparatus is providedwith legs or a ring support 56 or other such means on which the devicerides on surface 42 to avoid damage to grating 38 and to keep thedistance between the grating and the surface fairly constant.

FIG. 3B shows an alternative preferred embodiment of the inventionincluding direction detection by phase shifting of the local oscillatorand utilizing first order diffraction from the grating. Elements whichare functionally the same as those of FIG. 3B are given the samereference numerals in both FIGS. 3A and 3B. FIG. 3B shows a device 60 inwhich light from laser diode 32 is collimated by a lens 62 to strike agrating 38. Grating 38 is preferably mounted on a piezoelectric ring 64(which is in turn mounted on optical substrate 36). Excitation of ring64 adds a variable phase to the local oscillator (the light diffractedfrom grating 38) in order to allow for direction detection, as describedabove. In the embodiment shown in FIG. 3B, both the detection of thesignals used for translation and direction detection on the one hand andfor compensation detection on the other hand, are based on first orderdiffraction by grating 38, but with opposite sign. Preferably,anti-reflection coatings are used, where appropriate, to reduce unwantedreflections.

An integrated optical chip is the preferred implementation scheme sinceit can be manufactured in large volumes for a low cost. The figure showsonly one detector for a single direction, with preferably a seconddetector measuring the orthogonal direction. All of the opticalelements—lenses, grating, mirrors and pinholes—are etched into ordeposited onto the optical substrate and are either reflective ortransmissive according to functionality. The discrete components in thesystem—laser diode, detector and piezoelectric transducer—are mounted ontop of the chip. The electronic elements of controller 54 may also bemanufactured or placed on top of the chip.

It should be understood that the features of FIGS. 3A and 3B can bemixed and combined. For example if, in FIG. 3A, grating 38 is mounted ona transducer such as ring 64, then the result would be a deviceoperating in the specular reflection (zeroth order) mode with increaseddynamic range and possibly additional axial translation detection.Furthermore, it is possible to use an asymmetric grating in place ofgrating 38 and ring 64 of FIG. 3B for the purpose of directiondetection. For these and other preferred embodiments of the invention,combining various aspects of the invention will occur to persons skilledin the art.

FIG. 3C shows yet another method of determining direction, in accordancewith a preferred embodiment of the invention. The device 70, shown inFIG. 3C is similar to device 60 of FIG. 3B except that grid 38 is placedat the lower surface of chip 36 and piezoelectric ring 64 is replaced bya birefringent plate 66. Source 32 produces linearly polarized lighthaving a polarization which is at an angle of 45 degrees with thebirefringent axis of plate 66. Radiation which is reflected from thesurface passes through plate 66 twice and consists of two waves, eachhaving a polarization direction at a 45 degree angle with that of theradiation reflected from or diffracted from grating 38. These waves arealso at substantially a 90 degree phase difference with each other(depending on the properties of the surface).

In addition, a polarizing beam splitter 68 is preferably placed beforedetector 50. Its axis is such that one of the two halves of thereflected radiation passes through beam splitter 68 to detector 50 andthe other half is reflected to a detector 67. In addition, beam splitter68 directs half the radiation reflected or diffracted from grating 38 toeach of detectors 50 and 67. The resulting signals detected by detectors67 and 50 will have a phase difference of 90 degrees. The sign of thephase difference can be used to determine the direction of motion.

While the present invention is described above in various embodimentsfor solving the general problem of translation measurement, themethodology is applicable to a large number of products. One particularapplication of the optical translation measurement method of theinvention is a novel optical cursor control device (mouse) which derivesits translatatory information from movement on substantially any diffusesurface, such as paper or a desktop. One design for such a device, inaccordance with a preferred embodiment of the invention, is shown inFIG. 4. An optical mouse 80 comprises an “optical chip” 82 which ispreferably a device such as device 30 or device 60 or a variation ofthese devices. Chip 82 is mounted in a housing 84 and views paper 42through an optical aperture 86 in housing 84. Input and output leadsfrom chip 82 are preferably connected to a printed circuit board 88 orthe like on which are mounted electronic circuitry 90 corresponding tothe controller of devices 30 or 60. Also mounted on PC board 88 are oneor more switches 92 that are activated by one or more push-buttons 94 asin conventional mice. The mouse is conventionally connected to acomputer via a cable 96 or with a wireless connection.

The method of measurement in accordance with preferred embodiments ofthe invention described above allows for a wide dynamic range oftranslation velocities, covering all the required range for normaloperation of a mouse. Such a device can be characterized as a ‘padlessoptical mouse’ to provide orthogonal signals to move a cursor fromposition to position on a display screen in response to movement of themouse over any sufficiently diffusely reflective surface, such as paperor a desk top. Thus, special contrasting markings or special patternsare not necessary.

Mouse systems usually use mechanical transducers for the measurement ofhand translation over a surface (commonly a “mouse pad”). A need formoving-parts-free, reliable and accurate translation measurementtechnology for use in mouse systems is well acknowledged today. A fewoptical devices were developed, but still suffer from variousdeficiencies, such as a need for a dedicated patterned pad, lowtransducing performance or high cost.

An optical padless mouse according to one preferred embodiment of theinvention can be used in two ways, according to the user's convenience.It can be used as a “regular” mouse, whereby the mouse is moved on topof a surface, and its motion relative to that surface is measured. Itcan also be flipped over, if so desired, and instead used by moving thefinger along the device aperture. The motion of the finger relative tothe mouse body, which is now stationary, will be measured.

One such device 100 is shown in FIGS. 5A and 5B. FIG. 5A shows thatstructurally the device is similar to that of FIG. 4 (and the samereference numbers are used in the two Figs. for ease of comparison),except that buttons 94 are on the side of housing 84 in device 100. Inthe mode shown in FIG. 5A device 100 is stationary and it is used totrack the movement of finger 102 of an operator. It should be clear thatdevice 100 can be turned over and used as a mouse, in much the same wayas the mouse of FIG. 4. FIG. 5B shows a perspective view of the device,showing an optional switch 104 which is used to indicate if device 100is used as an ordinary mouse or in the mode shown in FIGS. 5A and 5B.Alternatively, such a switch may be a gravity switch mounted in thedevice to automatically switch modes. It is generally desirable to knowin which mode the device is operating since the direction of motion ofthe cursor is opposite for the two modes and usually, the sensitivitydesired is different for the two modes.

Furthermore, using a translation measurement device with a smallaperture, as in the present invention, and moving a finger along itsaperture, enables moving a cursor through measurement of the translationof the finger, much like a touch pad. This function may be termed“touch-point” and may be used in dedicated minute locations on keyboardsas well. This device would be identical to the device of FIG. 5 exceptthat the optical chip would be mounted in the keyboard as would theswitches. Also, an OTM “touch-point” may be used on the top of the mouseas an alternative to a scrolling wheel. “Clicks” may be detected, forexample, by bringing the finger into and out of range of the touchpoint.

This device can be used to replace pointing devices other than a mouse.For example, pointing devices used in laptop or palmtop computers.Virtually any one or two dimensional motion can be controlled using sucha device.

Currently, laptop computers pointing devices employ either a track ball,a touch pad, a trackpoint (nipple) or an attached mouse. These devicescarry diverse drawbacks. In particular, the track ball collects dustmuch like a regular mouse, the touch pad is sensitive to dampness andwas hailed unfriendly by many users, the trackpoint drifts when itshould be idle and the attached mice are delicate and require a desktopto work on.

The touch-point device is small in size, its working aperture can beless than 1 mm² and it provides high resolution and dynamic range. Thismakes it an ideal solution as a pointing device to be embedded in alaptop computer. The device is operated by moving a finger across theface of the aperture, in a somewhat similar manner to the use of a touchpad. The difference being, that the aperture is very small in sizecompared to the touch pad, it is free of problems like humidity anddampness and its reliability is expected to be high. In fact, evenseveral devices can be easily embedded in a single laptop or a palm top,including on keys, between keys, or next to the screen. Additionally, apressure sensitive device may be included under the touch point deviceand the sensitivity of the touch point made responsive to the pressureof the finger on the touch point.

In a preferred embodiment of the invention, two touch points areprovided, a first touch point and circuitry which moves a pointerresponsive thereto and a second touch point and circuitry which causesscrolling responsive thereto.

In a further preferred embodiment of the invention, the presentinvention can be used as an improved translation and/or velocitymeasurement system for a scanning-pen, capable of scanning lines of text(or any other pattern) and storing them, for downloading later to a PC,and/or for conversion to ASCII code using OCR software. An example ofsuch a device is shown in FIG. 6. A scanning pen 120 comprises a‘reading’ head with a one dimensional or two dimensional array of photodetectors (CCD array) 122 and a lens 123, wide enough to scan a typicalline height, and a lighting source 124 as in conventional light pens.The pen head also contains an optical translation measurement system 82in accordance with the invention, for one or two axes measurement of thetranslation of the pen head across the scanned paper and possiblyanother one to extract rotation information. The pen can then eitherstore the scanned line as a bitmap file (suited for hand-writing,drawings etc.) or translate it immediately through using internal OCRalgorithm to binary text. The stored information may be downloaded laterto a computer, palntop or phone, etc. For this purpose and for thepowering and control of the various devices in pen 120, it is providedwith a controller or microprocessor 128 and batteries 129.

The optical translation method of the present invention allows for thisdevice to be small in size, convenient to use, and accurate. The highaccuracy results from the inherent high accuracy of the method withrespect to current mechanically-based translation transducers and fromthe ease of measurement in two dimensions plus rotation. Similarcommercial devices today use a patterned wheel which is pushed againstthe scanned surface while scanning and rolled in order to measure thetranslation by detecting the rolled angle of the wheel. This techniqueonly detects the location along the line and not along its vertical axisand its relatively low accuracy limits the range of applications it canbe used for.

A further preferred application of the optical translation method anddevice of the present invention is a portable or a fixed device, forscanning signatures and relaying them to an authentication system.Similar in principal to the scanning pen, the signature reader containsa ‘reading’ head, with a one dimensional or two dimensional array ofphoto detectors (CCD array) It has an aperture wider than that of thescanning pen, to be able to read wider or higher signatures and containsan optical translation measurement device, for detection of the two axestranslations of the hand or instrument which is moving the device acrossthe scanned signature. The signature reader does not contain any OCR, asno text files are to be created. Instead, it is connected (throughdirect, hardwire line or wireless link, or through an off-line system),to an “authentication center”, where the scanned signature is comparedto a “standard signature” for validation. This device can be accurate,while cheap, small and easy to use.

A still further application of the devices and methods described aboveis in the field of encoders. The present invention can replace linearencoders and angular encoders, which generally require highly accuratemarkings on either an encoder wheel or on a surface, by a substantiallymarkless encoder. An angular encoder 130 in accordance with this aspectof the invention is shown in FIG. 7. Encoder 130 comprises a disk havinga diffusely reflecting surface 132 mounted on a shaft 131. It alsoincludes an optical chip 82 and controller 90, preferably essentially asdescribed above. Preferably, surface 132 is marked with one or tworadial marks 136 to act as reference marks for the encoder and forcorrection of errors which may occur in reading the angle during arotation. This mark may be read by optical chip 82 or by using aseparate detector.

A further embodiment of the invention is a virtual pen, namely a penwhich translates movement across a featureless page into positionreadings. These position readings can be translated by a computer intovirtual writing which can be displayed or translated into letters andwords. The computer can then store this virtual writing as ASCII code.Transfer to the computer may be either on line (using a wired orpreferably a wireless connection to the computer) or off-line whereinthe code or positions are stored in the “pen” and transferred afterwriting is completed. This embodiment of the invention provides acompact, paper-less and voice-less memo device.

In a typical fax/printer, the paper is moved in a constant speedrelative to the writing head with an accurate motor. The head releasesthe printed data line by line, in a correlated fashion with the speed ofadvancing paper. This method is both expensive, as it requires anaccurate motor and mechanical set up, and inaccurate, as the papersometimes slips in the device, thus the paper translation is not wellcorrelated to the printing device, resulting in missed or crooked lines.

With an optical translation measuring device, it is possible to detectpaper slippage, or even to eliminate the use of expensive accuratemotors, by measuring the paper advancement on-line. The printing deviceis then coordinated with the actual translation of the paper, thuscreating a highly accurate and economic system. Similarly, theseprinciples can be applied to a desktop scanner, where the printing headis replaced by a reading head.

FIG. 8 is a schematic of a motion sensor useful in a scanner, faxmachine or printer in which motion is only in one direction. Motiondetector 200 includes a source 202 which is fed to a housing 204 by afiber optic cable 205. The output of cable 204 is collimated by lens 206and illuminates a moving surface 208, through a grating 210. Lightreflected from grating 210 and surface 208 is collected by a fiber opticcable 212 which is placed at the focal point of lens 206. The output ofcable 212 is fed to a detector 214, for further processing as describedabove. Since the paper moves in only one direction, there is no need todetect the direction of motion of the paper.

In a preferred document scanner embodiment of the invention, the motiondetector measures the relative movement of a document, preferably,without utilizing any printing on the document, while a reading headreads printed information from the document. A memory receivesinformation from the printing head and stores it in memory locations,responsive to the measurement of movement of the document.

In a preferred printer embodiment of the invention the motion detectormeasures the movement of a sheet on which markings are to be made and amemory transmits commands to mark the paper, in accordance withinformation in the memory, responsive to the measurement of motion ofthe paper.

Either or both preferred scanner and printer embodiments of theinvention may be utilized in a facsimile machine in accordance withpreferred embodiments of the invention.

FIG. 9 is a simplified block diagram of typical electronic circuitry 140useful in carrying out the invention. A “primary” photodetector 142(corresponding, for example to detector 50 of FIGS. 3A and 3B) receiveslight signals as described above. The detector detects the light and theresulting signal is preferably amplified by an amplifier 144, band passfiltered by a filter 146 and further amplified by an amplifier 148 toproduce a “primary” signal. A compensating signal as detected, forexample, by photodetector 150 (corresponding to detector 52 in FIGS. 3Aand 3B) is subtracted (after amplification, by amplifier 152 andband-pass filtering by filter 154) from the “primary” signal in adifference amplifier 155 to remove residual low frequency components inthe primary signal. Preferably, band pass filters 154 and 146 areidentical. The resulting difference signal is amplified by a voltagecontrolled amplifier 156 whose gain is controlled by the output of a lowpass filter 153 (which is attenuated by an attenuator 158 optionallyadjusted during calibration of the system). The output of amplifier 156is fed to a zero crossing detector and counter 160 and (if a stationarynon-symmetric grating is used) direction control logic 162, whichdetermine the direction of translation of the surface. Alternatively,where a piezoelectric element 64 (FIGS. 3A and 3B) is used, a controlsignal corresponding to the frequency of displacement of the of theelement is fed to the direction control logic 162 where it is subtractedfrom the zero-crossing detector count.

For preferred embodiments of the invention, the wavelength of the lasersource is preferably in the infra-red, for example 1550 nanometers. Aspectral width of 2 nanometers is typical and achievable with diodelasers. A source power of 5 mW is also typical. A grating opening of 1.5mm by 1.5 mm and a grating period of 150 lines/mm are also typical. Thelaser source output is typically collimated to form a beam having adiameter of somewhat less than 1.5 mm and is typically incident on thegrating at an angle of 30 degrees from the normal. The optical substratemay have any convenient thickness. However a thickness of several mm istypical and the focal length of the lenses used is designed to providefocusing as described above. Typically, the focal length of the lensesare a few mm. Typically, pinhole 46 (FIGS. 3A, 3B and 3C) has a diameterof several micrometers, typically 10 micrometers. It should beunderstood that the above typical dimensions and other characteristicsare provided for reference only and that a relatively wide variation ineach of these dimensions and characteristics is possible, depending onthe wavelength used and on other parameters of the application of theoptical chip.

The present invention has been described in conjunction with a number ofpreferred embodiments thereof which combine various features and variousaspects of the invention. It should be understood that these featuresand aspects may be combined in different ways and various embodiments ofthe invention may include one or more aspects of the invention. Thescope of the invention is defined by the following claims and not by thespecific preferred embodiments described above.

As used in the following claims, the words “comprise” or “include” ortheir conjunctions means “including, but not necessarily limited to.”

What is claimed is:
 1. A method for determining the relative motion of asurface with respect to a measurement device, comprising: illuminatingthe surface with incident illumination, having a coherence length, suchthat the illumination is reflected from portions of the surface, whereinat least part of at least one of the incident and reflected illuminationpasses through a partially transmitting object that is part of themeasuring device; detecting the illumination reflected from the surface,to generate a detected signal; and determining the relative motion ofthe surface parallel to the surface, from the detected signal, whereinthe object and the surface are situated within a distance that is lessthan the coherence length of the detected illumination.
 2. A methodaccording to claim 1 wherein the transmission of the object is spatiallyvarying.
 3. A method according to claim 1 wherein the object ispartially reflecting and wherein part of the incident illumination isreflected or diffracted by the object, as a reference illumination andwherein detection of the illumination is coherent, utilizing saidreference illumination.
 4. A method according to claim 2 wherein theobject is partially reflecting and wherein part of the incidentillumination is reflected or diffracted by the object, as a referenceillumination and wherein detection of the illumination is coherent,utilizing said reference illumination.
 5. A method according to claim 2wherein spatially varying comprises periodic spatially varying.
 6. Amethod for determining the relative motion of a surface with respect toa measurement device comprising: placing a partially reflecting object,which is part of the measuring device, adjacent to the surface;illuminating the object with incident illumination such that part of theincident illumination is reflected or diffracted by the object, as areference illumination and part is reflected from the surface;coherently detecting the illumination reflected from the surfaceutilizing the reference illumination, to generate a detected signal; anddetermining the relative motion of the surface parallel to the surface,from the detected signal; wherein the surface has no markings indicatingposition.
 7. A method according to claim 6 wherein the object is apartially transmitting object and wherein at least part of at least oneof the incident and reflected illumination passes through the object. 8.A method according to claim 7 wherein the reflection of the object isspatially varying.
 9. A method according to claim 8 wherein spatiallyvarying comprises periodic spatial variation.
 10. A method according toclaim 6 wherein the reflection of the object is spatially varying.
 11. Amethod according to claim 10 wherein spatially varying comprisesperiodic spatial variation.
 12. A method according to any of thepreceding claims wherein the object is a grating.
 13. A method accordingto claim 12 wherein the grating is placed sufficiently close to thesurface such that the surface is in the near field of the grating.
 14. Amethod according to claim 12 wherein the grating is placed sufficientlyfar from the surface such that the surface is outside the near field ofthe grating.
 15. A method according to any of claims 1-11 wherein thedetected illumination is at least partly coherent.
 16. A method fordetermining the relative motion of a surface with respect to ameasurement device comprising: placing an object comprising a grating,which is part of the measuring device, adjacent to the surface;illuminating the grating with incident illumination such that at leastpart of the illumination is incident on and reflected from the surface,wherein at least one of the incident and reflected illumination passesthrough the grating; detecting the illumination reflected from thesurface; generating a signal in response to the reflected illumination;and determining the relative motion of the surface parallel to thesurface, from the detected signal, wherein the surface is in the nearfield of the grating.
 17. A method according to any of claims 3, 6 or 16wherein the illumination reflected from the surface is frequency shiftedfrom that of illumination reflected from or diffracted by the object andwherein determining the motion comprises determining the motion based onthe frequency shift.
 18. A method according to any of claims 1, 6 or 16wherein determining the relative motion comprises determining variationsin the amplitude of the signal with position.
 19. A method according toclaim 18 wherein the motion is determined from zero crossings of thedetected signal.
 20. A method according to any of claims 1, 6 or 16wherein the object has a transmission characteristic that is spatiallynon-symmetric and including: determining the direction of the relativemotion based on the detected signal.
 21. A method according to any ofclaims 1, 6 or 16 and including determining the magnitude and directionof the translation utilizing two detectors which produce differentdetected signals depending on the direction of the translation.
 22. Amethod according to claim 21 and including determining the direction oftranslation from the sign of a phase difference between the differentdetected signals.
 23. A method according any of claims 1, 6 or 16wherein the illumination is perpendicularly incident on the surface. 24.A method according to any of claims 1, 6 or 16 wherein the surface is anoptically diffusely reflecting surface.
 25. A method according to claim1 or claim 16 wherein the surface has no markings indicating position.26. A method according to any of claims 1, 6 or 16 wherein theillumination comprises visible illumination.
 27. A method according toany of claims 1, 6 or 16 wherein the illumination comprises infra-redillumination.
 28. A method according to any of claims 1, 6 or 16 andincluding determining motion in two dimensions transverse to thesurface.
 29. A method according to claim 28 and including determiningmotion in a direction perpendicular to the surface.
 30. A scanner forreading a document by movement of the scanner over the documentcomprising: an optical reading head which detects patterns on thesurface of the document; and an optical detector which determines themotion of the scanner as it is translated across the surface of thedocument, wherein the optical detector utilizes the method of any ofclaims 1, 6 or 16 to determine the translation.
 31. An optical mousecomprising: a housing having an aperture facing a surface; and anoptical motion detector which views the surface through the aperture,wherein the optical motion detector utilizes the method of any of claims1, 6 or 16 to determine the translation of the housing with respect tothe surface.